1. Field of the Invention
This invention relates generally to implantable medical devices, and, more particularly, to methods, apparatus, and systems for providing an assessment relating to the condition of a lead coupled to the implantable medical device.
2. Description of the Related Art
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is hereby incorporated in its entirety herein by reference in this specification. Electrical stimulation of the vagus nerve (hereinafter referred to as vagus nerve stimulation therapy or VNS) may be provided by implanting an electrical device underneath the skin of a patient and performing a detection and electrical stimulation process. Alternatively, the system may operate without a detection system once the patient has been diagnosed with epilepsy, and may periodically apply a series of electrical signals to the vagus (or other cranial) nerve intermittently throughout the day, or over another predetermined time interval.
Generally, therapeutic electrical stimulation is delivered by the implantable device via a lead, which is coupled to one or more electrodes coupled, in turn, to a target location of the patient's body. A plurality of electrodes that are associated with an implantable medical device are generally operatively connected to the implantable device via individual leads. A number of leads may project from the implantable device onto various portions of a patient's body. For example, a number of electrodes may be attached to various points of a nerve or other tissue inside a human body. Occasionally, problems with the lead and/or electrodes may occur. These problems may include a malfunction or damage of the lead and/or electrode, or a change in the tissue surrounding the implanted lead and/or electrode.
Often, various electrodes are implanted in contact with target portions of the human body, such as a vagus nerve, in order to deliver electrical signals to provide therapy or to monitor signals. Subsequent to the implanting of the implantable device, the associated leads, electrodes and/or connections between the electrodes and the implantable device may deteriorate over time. Additionally, changes in the tissue surrounding the lead and/or electrodes may cause electrical variations experienced by the implantable device system, which may affect the operation of the leads and electrodes themselves. Electrical characteristics associated with the leads and electrodes carrying the stimulation signal or monitored signal may deteriorate over time, thereby altering the operation of the implantable device. Furthermore, physiologic changes in the human body may also affect the operation of the implantable device since these changes may affect the electrical characteristics experienced by the lead and/or the electrodes.
State-of-the-art assessment of lead condition may include measuring an impedance between a plurality of electrodes[r1]. A rise in the lead impedance may provide an indication that the lead condition has changed. This may be caused by various factors, such as deterioration of the lead, deterioration of the electrode, deterioration of connections between the electrode and the implantable device, and/or the physiological changes in the human body. Based upon the impedance measurements, state-of-the-art technology calls for assessing or concluding that there may be lead problems. However, a simple rise in lead impedance may not necessarily reflect actual lead problems. For example, physiological impedance changes may provide a false negative indication that there are lead problems. Additionally, the lack of a rise in lead impedance may provide a false positive indication that there are no problems with the leads or electrodes. For example, a lead problem may be masked by an apparent lack of change in lead impedance. This apparent lack of change in the lead impedance may actually be an increase in lead impedance (e.g., due to lead/electrode damage) being masked by a reduction in the physiologic impedance. The reduction in the physiologic impedance may counter-balance the rise in the electrode or lead impedance that may have been the result of actual damage. However, the result causes a false assessment of the actual condition of the lead and/or electrode. This could lead to improper delivery of therapeutic stimulation or improper assessment of monitored signals by the implantable device.
Other problems with the state-of-the-art include the fact that the insertion or placement of the lead and electrodes into the patient's body may be implemented incorrectly. For example, the insertion of the leads may be reversed compared to the originally intended position of the electrodes and/or leads. For example, the lead/electrode in a set that was originally intended to be positioned proximal to the implantable device may be inadvertently positioned in a distal position, while the intended distal electrode may inadvertently become the proximal electrode. Therapy stimulation being provided may be ineffectively administered or monitored signals may be errantly assessed due to the various errors described herein.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.